This invention relates to filter cartridges.
A known filter cartridge is described in U.S. Pat. No. 3,531,920 and has a block-like form and comprises a filter element in which the filter material is folded in a zig-zag fashion. The filter material is provided with offsets which extend out of the plane of the filter material on both sides in parallel lines, i.e., as raised portions and as recesses. The surfaces of the offsets of two adjacent fold walls are in contact with each other when the filter material is folded. Each offset which lies perpendicularly in relation to the crease in the filter material which creates a fold becomes wider in the direction of increasing height or depth of the offset. In a fold wall adjacent offsets protrude in opposite directions; i.e., as raised portions and recesses.
The offsets in the fold walls are arranged in such a way that an offset in the form of a raised portion on one fold wall is in contact with an offset in the form of a raised portion on the adjacent fold wall. Due to the folding in a zig-zag fashion the offsets of the fold walls are in contact with each other and are supported in the region of the contact area.
In this way the spacing between two adjacent fold walls decreases uniformly in the direction of flow until the medium to be filtered passes through the filter media from when on the spacing increases uniformly.
A drawback of this arrangement is that the width of the offsets varies in the direction of flow so that the medium to be filtered is deflected sideways. The resistance to flow of the medium to be filtered increases and a greater throughput is therefore required. The increasing width of the offsets also decreases the available filter area in the region of the fold walls.
In addition the width of the channel which is formed between two sets of adjacent offsets increases in the direction of flow. Due to the accompanying change in cross-section the resistance to flow is again increased in an undesirable fashion. To be able to filter larger quantities of a medium economically one must try to minimize the amount of energy required as well as taking into consideration the fact that motors are louder the more powerful they have to be.